A WebP can be larger than its PNG source because WebP supports both lossy and lossless encoding, and neither mode guarantees a smaller file for every image. The most common culprit is using lossy WebP for a simple, few-color graphic; another is converting an already-lossy image to lossless WebP. Check the mode and source type before changing quality settings.
Why a WebP can be larger than its PNG source
“WebP” names an image format, not a single compression method. It supports lossy and lossless compression, as well as transparency and animation. The mode selected by the converter—and how well that mode suits the image—determines whether the result is smaller.
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Google reports that lossless WebP images are 26% smaller on average than PNGs, but that aggregate format claim is not a guarantee for an individual image. Google’s WebP FAQ also documents cases where conversion makes a file larger: Can a WebP image grow larger than its source image?
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A few-color PNG was encoded as lossy WebP
Icons, diagrams, screenshots, and other artwork with flat colors or sharp boundaries can be awkward inputs for lossy WebP. Google explains that a PNG’s lossless ARGB data may be converted to YUV420, a process that can introduce colors absent from the original small palette. Those additional colors can be harder to compress, so the lossy WebP can end up larger.
An already-lossy image was encoded as lossless WebP
Lossless WebP preserves the decoded pixel values it receives. If the source was already compressed lossily, its artifacts and irregularities must be represented exactly in the lossless output. That can cost more bytes than keeping the original lossy encoding. Google gives JPEG-to-lossless-WebP and JPEG-to-PNG as examples of this general pattern.
The lossy quality target is higher than the source needs
A lossy conversion can grow if its quality target is set above the source’s effective quality. Google illustrates this with a JPEG saved at quality 80 converted to WebP at quality 95. In that situation, the encoder is being asked to retain more detail, which can increase file size.
How to diagnose and fix it
- Check the output mode. In your converter, find whether the export is set to lossy or lossless WebP. Do not assume the mode from the file extension.
- Identify the image type. If the PNG is a logo, icon, screenshot, or other few-color graphic, try a lossless WebP export rather than lossy encoding.
- Check whether the input was already lossy. If it came from a JPEG or another lossy source, lossless WebP may preserve its existing artifacts at a larger byte cost. Compare lossy WebP settings if exact pixel preservation is not required.
- Adjust lossy output deliberately. If lossy WebP is the right mode but the file is too large, lower the quality target in small increments. Google also recommends using cwebp’s
-sizeoption, or the equivalent API, to target an output size. A size target does not itself guarantee equivalent visual quality. - Compare at the intended use. View both files at the dimensions where they will appear. Inspect fine edges, color boundaries, and transparency; a smaller byte count is not proof that the images look equally good or preserve the same alpha details.
Choose the mode by what must be preserved
There is no universally best WebP setting. For each image, weigh whether you need lossless pixel preservation, how it looks at its intended display size, whether transparency must remain, and the final file size. WebP supports both compression modes and alpha transparency, but the best trade-off depends on the image and the conversion settings. See Google’s WebP format overview for the published average size comparison; treat it as an aggregate claim, not a prediction for your particular PNG.
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